Up until a couple of days ago, that was a theoretical assumption. Then something happened that validated my assumption.
It's not from Google or Amazon or eBay. It's from my ISP, the one that was bought by BestBuy some time ago. I have an email account with them that I use everyday from Thunderbird (or Icedove on Debian). They do spam blocking on the server side, and once in a month or so, I would check the spam folder on the server side to examine if there were any false positives.
I do that through a browser based interface. And on my most recent visit, there was a note saying something like "if you use Firefox, you might like our new RIA version of the webmail."
And I clicked on the link, which lead me to a XUL based version of their mail client. And guess what? It's better than the HTML interface. As far as I'm concerned, the XUL interface offers one feature that beats the HTML interface: I can now drag the table column divider to see more of the Subject lines. In the HTML version, here's what I see:
*** [Spam score 20.6] ***: Get...
and I can't make it show more of the subject. With the XUL version, I see:
*** [Spam score 20.6] ***: Get meds from Canada
and I can always make it show more of the subject line if it is longer.
You can't believe how big a usability win it is for XUL.
I would choose a RIA application over HTML any day.